Rendering Adds Realism

Last time (January 2002), we used
AutoCAD's standard visualization functions to go from a basic wire frame
through a simple hidden-line removal (figure 1) and on to the shaded image
of figure 2. This month, we'll dabble with AutoCAD's rendering operations
to see how easy it can be to produce a photorealistic image such as figure
3. No, figure 3 is not the same object as figures 1 and 2. Rendering is
good, but not that good.

Figure 1. This part
is a wire frame drawing with hidden lines suppressed.

Figure 2. Now the
part has Gouraud shading applied.

You light up my life
Figure 4 shows a glass of wine sitting on a table, displayed with Shademode
set to Gouraud. If you want to follow along, this drawing is available
at www.cadalyst.com/solutions/ lc/0302lc/0302lc.dwg.

Figure 3. A photorealistic
ray-traced rendering of a glass of wine on a table.

Figure 4.Gouraud
shading of a glass of wine.

All light is provided by ambient light, along with a single fixed light
coming from over your left shoulder. Ambient light is the kind of soft,
shadowless light that you get on a cloudy day. The brightness of objects
or parts of objects is determined solely by the angle between your viewpoint
and the fixed light. This lighting, though not too spectacular, is adequate
for many objects.

To get really impressive visualizations, however, you need to explore
the rendering functions in AutoCAD.

Figure
5. Dialog box for AutoCAD's Light command.

Figure
6. New Point Light dialog box.

Select View | Render | Light to bring up the dialog box in figure 5.
Here you control the brightness of the ambient light and add specific
light sources. Let's add a new light.

First, make sure the box beside the New button shows Point Light, then
click on the New button. This brings up another dialog box (figure 6),
in which you define the light.

Your light needs a name with a maximum length of eight characters. While
you are here, turn on the Shadows box. Now go to the Position area and
click the Modify button.

The dialog box collapses, and AutoCAD asks you to specify a position
in the drawing for the light. You can use any normal AutoCAD method, such
as typing in a point triplet, or you can use object snaps. For our sample
drawing, the origin is at the left front corner of the table, so I placed
the light at 100,-20,40.

When you finish the operation, AutoCAD places a small block with an attached
attribute in the drawing. If you want to move the light later, you can
use standard AutoCAD Move operations. Note that the coordinates of a light
when you create it, or later if you modify it, are based on the UCS that
is current at the time of creation or modification.

Once you place the light, the previous dialog box returns. Select OK
to return to the Lights dialog box, then click OK again to return to the
drawing.

The first thing you notice is that nothing has changed. If you set Shademode
to Gouraud, your image still looks like figure 5. To see the effect of
adding the light, you must use the Render command.

Figure 7. Render
dialog box.

Figure 8. A simple
rendering with shadows turned on.

Click on View | Render | Render to bring up the dialog box in figure
7. Make sure you select the following options:

Rendering Type: select Photo Real from the scroll
list.

In the Rendering Options area, turn on Smooth Shade and Shadows.

Click on Background. In the dialog box that appears, turn off AutoCAD
Background and turn on Solid. Now set all three primary colors to
1.0, then click OK.

Click Render, and after a short pause the screen looks like figure
8. Note that anything that changes the screen view, such as scrolling
with the mouse wheel or slider bars, redraws the screen, which makes
your rendering disappear. I'll tell you later how to save your rendering.

The shadow knows
Keep in mind these significant points about lights, shadows, and rendering.

First, as noted earlier, when you create a light, you must tell it that
it is capable of casting shadows. In addition, in the Render dialog box
you must turn the Shadows option on and set the render type to Photo Real
or Photo Raytrace.

Next, you often need to increase the Ambient light in the Render dialog
box. The default intensity for individual lights is so high that it often
swamps the ambient.

You can create three types of lights in AutoCAD. We have already used
a Point light. This radiates light outward in all directions, like a light
bulb.

You can also create a Spotlight, which projects a cone of light. You
can specify the cone angle for the central hot spot and for the falloff
zone around the hot spot.

The final light type is a Distant light. This is a light that is so far
away (say 93,000,000 miles) that its light rays are effectively parallel.
The real fun with this light is that it offers Sun Angle options. It is
used mainly in architectural applications. You specify the location of
your building either by latitude and longitude or by city name. You can
then vary the date and time to see how shadows change during the year.
You may want to be careful using the city name option, however. I am aware
of at least two hiccups in the listit doesn't include Vancouver,
BC, Canada, and it locates Victoria, BC, Canada, about 300 miles too far
south.

Placement of lights is more of an art than a science. A good starting
point is to use the method that portrait photographers use. They usually
start with a brighter light higher and behind the camera so it shines
over one of their shoulders. A second, dimmer light is placed lower and
to the other side, but still behind the camera, so it fills in and softens
the shadows.

I'm a material guy
The next major step in realistic renderings is the application of materials.

Figure 9. Materials
(Rmat) dialog box.

Figure 10. Materials
Library (MatLib) dialog box.

Click on View | Render | Materials to bring up the dialog box in figure
9. Click on the Materials Library button near the center to bring up the
dialog box in figure 10. Material definitions reside in an external file,
and just like linetype definitions, you have to specifically import the
ones you want to use. Scroll down the list until you find Glass. Select
it, then select <-Import. Click OK to return to the Materials dialog
box (figure 9).

In the Materials dialog box, make sure Glass is highlighted, then select
Apply. The box collapses, and you are invited to select one or more objects.
Select the wine glass, then press <Enter>. When the dialog box returns,
click OK.

Figure 11. Rendering after Glass material
was applied to the wine glass.

Now, run View | Render | Render again. Make sure the Materials option
is turned on, then select Render. Magic! As figure 11 shows, the glass
is now transparent. Repeat these steps to apply WoodMed. Ash to
the table.

While you are in the Materials dialog box, select New to create a new
material called WineRed. The left side of this dialog box lets you
select an attribute to define, and the definition for each attribute is
set in the center section of the dialog box. Use the following settings:

Color/Pattern value 1, turn off By ACI, and set Red
to 1.0

Roughness 0.5

Transparency 0.5

Refraction 1.0

Select OK, then attach this material to the wine. Now run Render, but
this time change the Render Type to Photo Raytrace.

The rendering takes somewhat longer, but eventually produces the image
in figure 3.

Notice how the wood grain correctly differentiates between face and edge/end
grain and the glass and the wine cause proper reflections and refractions.
Just for fun, render again using Photo Real. Note the differences in time
and appearance.

So now that you have your rendering just right, how do you save it? There
are several ways to capture screen images, but the easiest is to click
on Tools | Display Image | Save to start the SaveImg command. Choose a
file format, then select OK. You are next asked for a filename. I usually
use Caesar, because I like to

Render unto Caesar
You can use this method to capture any screen shot in AutoCAD, not just
a rendering.

This brief introduction to some of AutoCAD's visualization features should
whet your appetite to do more exploring and experimenting.

And now for something completely different
When two full moons fall in the same calendar month, the second one is
called a blue moon because old calendars colored it blue to differentiate
it from the first one. This happens about once every 16 months.

Halloween 2001 was a blue moon on the west coast of North America for
the first time in 46 years, and it won't happen again until 2020. Because
of time zone differences, Halloween was not a full moon on the east coast.

Twice in a blue moon. Coincidentially with the blue moon on October
31, Captain LearnCurve and his lovely wife became grandparents again.
Two-year-old Rebecca has a baby brother, Bryan Robert Milne, 7 pounds,
15 ounces, and another baby brother, Brodie Kenneth Milne, 7 pounds, 6
ounces.

About the Author: Bill Fane

Autodesk Technical Evangelist Lynn Allen guides you through a different AutoCAD feature in every edition of her popular "Circles and Lines" tutorial series. For even more AutoCAD how-to, check out Lynn's quick tips in the Cadalyst Video Gallery. Subscribe to Cadalyst's free Tips & Tools Weekly e-newsletter and we'll notify you every time a new video tip is published. All exclusively from Cadalyst!Follow Lynn on Twitter

Poll

How many hours per week do you spend creating renderings and animations? (Not counting unattended processing time.)